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Difference Between Screening & Diagnostic Tests

Submitted on March 27, 2012
While some medical tests may be used for both screening & diagnostic purposes, the terms are not interchangeable.
Screening Tests Vs. Diagnostic Tests

Very simply put, a diagnostic test in medicine is any kind of test used to aid in the diagnosis of a suspected disease or condition. It is usually required after the individual has exhibited symptoms of a particular disease or condition.

On the other hand, a screening test helps to identify individuals who are not exhibiting any symptoms (asymptotic) and determine if they are at risk of developing a disease or condition.

Diagnostic test are usually performed after a positive screening test to establish a definitive diagnosis. They are also offered to individuals who have some indication, be it a symptom, sign or a history of a particular disease or condition.

Screening tests are usually used for individuals who are considered to be at a high risk of developing a disease or condition. They may not as yet have the disease or its precursors. The test results guide the doctor in determining whether or not to ask for a diagnostic test to confirm his suspicions. In other words, a positive screening test indicates the possibility that the individual in question may have the suspected disease or condition and this can then be confirmed by a diagnostic test.

For example, when a woman undergoes a routine mammogram without having any of the symptoms of breast cancer, it is considered to be a screening test. If the mammogram detects any abnormalities such as a growth, a biopsy may be performed. The excised lump will then be sent to the laboratory to test for any malignancies. The biopsy and laboratory test together will be considered to be a diagnostic test that will indicate whether or not the woman has cancer.

Characteristics of a Diagnostic Test

  • The cutoff for a positive result is precisely defined, with stress being laid on diagnostic precision and accuracy
  • They cost more because of the accuracy that is required
  • The test provides for a definitive diagnosis
  • The test is often invasive, such as a lumbar puncture to test for meningitis
  • They are used for individuals who are symptomatic

Characteristics of a Screening Test

  • The cutoff for a positive result is extremely sensitive and may result in numerous false positives
  • The cost is usually low, since the accuracy is low
  • The test results determine the level of risk and whether a diagnostic test is required
  • The tests are usually non-invasive
  • They are used for high risk individuals who are asymptomatic

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